The Biostatistical and Data Management Core, will provide the statistical expertise and data entry and data management support needed by Chronic Fatigue Syndrome Clinical Research Center (CFS CRC) investigators. Its specific aims are to 1) provide consultation and collaboration on study design, methodology, and data analysis; 2) assist with the design of study forms and the evaluation of pre-testing and pilot data; 3) perform and supervise data entry; 4) maintain data bases and perform data management and quality control procedures; and 5) collaborate in the preparing and writing of manuscripts. The establishment of a Biostatistical and Data Management Core will allow new investigators to obtain valuable advice on CFS-related Projects, and established investigators involved in the CFS CRC to get advanced biostatistical consultation and evaluate novel approaches to research relevant to this CRC. A particular strength of this Core will be the availability of methodologists and analysts who have worked with the CFS CRC over the preceding 4 years, have gained extensive knowledge about the problems encountered in the classification of CFS and the appropriate biostatistical methods required to analyze complex data produced by this CFS CRC.